Lithostratigraphy and sedimentology of the chalk in the North Downs, Southern England
The aim of this project is to establish a lithostratigraphic scheme for the North Downs Chalk and to study its sedimentology. Detailed field studies were made on the Chalk and where necessary supplemented by the analyses of insoluble residues, mostly by the use of X-ray diffraction. A lithostratigraphic scheme is proposed in which the North Downs Chalk is subdivided into five Formations. In ascending order these are the East Wear Bay Chalk Formation, Abbots Cliff Chalk Formation, Dover Chalk Formation, Ramsgate Chalk Formation and Margate Chalk Formation. A marker bed framework has also been developed and used to correlate the Chalk sections throughout the North Downs. The sedimentological study has shown that the Chalk was deposited cyclically. The cycles show a fining upward texture and have an average thickness of approximately 2m. A standard cycle is defined, which is supported by the use of facies relationship diagrams. Scour structures are recognized in association with discontinuity surfaces, which mark the base of the fining upward cycles. These structures are compared with the longitudinal furrows found in modern tidal shelf seas. They are thought to have been formed by the activity of tidal currents within the Chalk sea. The variation in grain size within the fining upward cycles is interpreted as the result of a gradual decrease in the velocity of the tidal currents. The contiguous development of the fining upward cycles throughout the Chalk indicates that repeated changes in current velocity occurred throughout the period of Chalk deposition. A study of marl bands, mostly by the X-ray diffraction analysis of insoluble residues, indicates that they are probably the product of airbourne volcanic ashfalls. The resultant comparatively large detrital component makes them lithologically distinct features which are valuable isochronous marker beds.